Fix create collection coordinator and re-enable jstests/core/timeseries/timeseries_user_system_buckets.js when the TrackingUnsplittableCollections feature flag is enabled.

The test attempts the following:

• create a collection (non timeseries) db.system.buckets.coll through insert
• create a collection (non timeseries) db.coll through insert

The test expect the 2 collections to be treated independently, without assuming db.coll and db.system.buckets.coll to be the same. This is true for replica set, but for sharded cluster we currently do assume the 2 collections are the same. 
This wasn't a problem until SERVER-81190: now collections are tracked by the CSRS. A create on db.system.buckets.coll will register the collection, causing the test to fail